This invention relates to an improved mode-locked laser.
Prior art locking of longitudinal modes in a continuous wave laser is commonly accomplished by intra-cavity loss or phase modulation with an acousto-optic cell as described in U.S. Pat. Nos. 3,947,780 of Rice et al. and 3,648,193 of Foster et al. The acoustic standing waves function as an optical shutter, diffracting sufficient light to maintain the laser below oscillation threshold except during nodes or nulls. If the time between the acoustic nulls (shutter open) corresponds to the laser round-trip time, the laser axial modes are locked to form a temporally compressed packet of circulating energy.
In order to achieve low-loss modulation, the acoustic nulls formed by interference between forward and reflected waves within the crystal of the cell must be deep. This constraint requires a high acoustic Q in the cell configuration and this in turn depends upon both low acoustic attenuation and, ordinarily, an acoustic round-trip path length which is an integral number of acoustic wavelengths. Since the cell path is temperature dependent, the latter condition is difficult to maintain. If the mode-locking drive frequency for the cell is externally constrained, the fabrication of the device is further complicated.
This invention is directed to a solution of this problem.